This illustration provided by NASA and based on Cassini spacecraft measurements shows the possible interior of Saturn's moon Enceladus, an icy outer shell and a low density, rocky core with a regional water ocean sandwiched in between the two at southern latitudes.

And, of course, what is most exciting to researchers is that where there's water — particularly liquid water — there just might be life.

As NPR's Nell Greenfieldboyce reports, NASA's Cassini probe has been studying Saturn and its moons for about a decade, and it transmitted images of Enceladus in 2005 that looked promising:

"Water vapor and ice was spewing out of its south polar region, where scientists saw long fractures that they nicknamed tiger stripes," she says.

Now, Nell says, there's "evidence that suggests the presence of an ocean at least as big as Lake Superior."

It was an unexpected speed bump on Cassini's trajectory that provided the final clue for scientists. They analyzed data from flybys of Enceladus in 2010 and 2012 that brought the probe within 60 miles of the surface — where Cassini experienced slight changes in its trajectory caused by fluctuations in the moon's gravity field.

The only good explanation for that turned out to be the presence of a large body of liquid water under the moon's south pole, says David Stevenson, professor of planetary science at Caltech and a co-author of the study that appears in Science.

"For the first time, we have used a geophysical method to determine the internal structure of Enceladus, and the data suggest that indeed there is a large, possibly regional ocean about 50 kilometers below the surface of the south pole," he says. "This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole."

"Enceladus has excited scientists because the vapour plumes from the south pole are known to contain organic molecules. These, along with basic elements, a source of heat and liquid water, make Enceladus a prime candidate in the search for alien organisms."

Jonathan Lunine of Cornell University, one of the study's authors, says we don't know the temperature of Enceladus' subterranean ocean today, "[but] it's conceivable that it was warm enough, with circulation of water coming from the silicate core as well, to allow life to form even if today that ocean is maintained by antifreeze and is slightly below the freezing point."

The "antifeeze"? Salt, Lunine says.

Water on Enceladus sets up an interesting dilemma for planetary scientists: It's long been thought that Jupiter's Europa was the logical target for an eventual probe to look for subsurface aquatic life. Now, Enceladus, though much more distant, might be the front-runner, says Carolyn Porco, the leader of Cassini's imaging team.

Europa's ice crust is thought to be at least 6 miles thick — presenting a huge challenge for reaching the liquid interior. While the ice layer on Enceladus is even thicker (at least 18 miles), there might be an easier way, Lunine says.

He says a spacecraft with instruments to detect organic material could be flown through the plumes being ejected from the moon's "tiger stripe" fractures. The spacecraft could test for a "menu of molecules" associated with an advanced biological system, he says.

Porco agrees. "Enceladus has the most accessible extraterrestrial habitable zone," she says. "This place is really where we should be going."